As plastic comes out of the mold it dries. As it dries it changes shape. The top may pop up or sink down which changes the nose angle of the disc. Even tiny changes in shape change how the disc flies. And a disc flies differently at different speeds and with different degrees of flutter, so a disc may be changed but not noticeable by you...

I have to make a clarification, and edited Marks' statement according to my understanding...discs are not made of epoxy (which dries), but rather they are made of thermoset plastics, with compositions derived from thermopolyurethane, polyethylene, etc. (vibram rubber is an exception). Therefore this should read (changes highlighted in bold):

Change the plastic and you change the disc.

As plastic comes out of the mold it cools. As it cools it changes shape by shrinking. Shrinkage is non-uniform owing to the non-simple geometry of the disc. The top may pop up or sink down which changes the nose angle of the disc. Even tiny changes in shape change how the disc flies. And a disc flies differently at different speeds and with different degrees of flutter, so a disc may be changed but not noticeable by you.

Also, I completely disagree with this statement...

Mark Ellis wrote:Plastic formulas are constantly tweaked by manufacturers and no one knows how the next formula will fly-until it is thrown. So sometimes certain trends are noticed: a particular mold comes out more of less stable in certain kinds of plastic, but this is only a general rule and no true certainty exists until YOU throw a disc and learn what it does.

It isn't true that you can't predict the shapes/properties of plastics processed by injection molding, there is an entire field of engineering that is devoted to exactly this task. Entire suites of industrial software have been written to do exactly this job. Disc manufacturers simply don't use it because they don't want to pay for it, and probably the market isn't anyways big enough to justify the expense.

Being a practical guy (and not a scientific guy), did JHern just agree with me?

Plastic pellets are heated until they melt with other types of plastic pellets similarly melted together til they all mix in a kind of plastic soup. Then that soup is spit into a mold where it dries ( er.. skrinks or becomes non-soupy and more solid-like). From soup to solid is where the change of shape happens. I may be omitting or massaging some technical terms.

Disc golf is such a tiny, tiny (really darn small) part of the plastic market that the big plastic manufacturers don't engineer plastics for discs. Rather disc companies try out different kinds of plastic ( which are themselves ever changing) then blend them with other kinds of fancy, high-tech, ever changing plastics hoping to make discs which the disc golf world will love. So if a company bought enough plastic pellets of one type to last for a year by the time they needed to reorder that exact plastic may no longer be on the market, replaced with the newest, greatest, improved stuff which may be better for a car bumper but not as good for a driver.

So when the disc manufacturer buys the newest order of pellet and then mixes it with 3 parts of X and 7 parts of Y and 2 parts of Q plus weighting and coloring agents, NO ONE knows how it will fly until it is thrown. Oh, by the way, the next run which happens to have the exact plastics and the exact formulas as the last won't be run under the same atmospheric conditions. So perhaps with enough time, money and effort software COULD be developed to predict performance. But since no one wants to spend megabucks for a disc it ain't gonna happen soon.

The top may pop up or sink down which changes the nose angle of the disc.

Should probably have a "and you can't know for sure how either affects the angle" or some other clarification like that at the end, since there are some discs that "clearly" seem to go up in PLH when they dome up (early Axes, Vectors), and likewise some that seem to go down in PLH when they dome up (Firebirds, DD2s, at least so as far as I've seen). It would be way too easy if discs behaved consistently like that. =)

Parks wrote:If the posts on this forum are any indication, the PD is like a Teebird with sunshine coming out of its butthole so hard that it flies faster.

Mark Ellis wrote:Plastic pellets are heated until they melt with other types of plastic pellets similarly melted together til they all mix in a kind of plastic soup. Then that soup is spit into a mold where it dries ( er.. skrinks or becomes non-soupy and more solid-like). From soup to solid is where the change of shape happens. I may be omitting or massaging some technical terms.

Going from liquid to solid is not the same thing as drying. Drying implies there something that gets out of the liquid so that it "dries up", usually if not always a solvent of some kind (water is a pretty darn common solvent =) ). The plastic used in discs doesn't have anything like that, it just cools down to a solid state (water doesn't "dry" when it goes solid i.e. freezes, neither does molten metal dry when it solidifies). I think that was the gist of that particular point. =)

Parks wrote:If the posts on this forum are any indication, the PD is like a Teebird with sunshine coming out of its butthole so hard that it flies faster.

Plastics are somewhat complicated, the transition from solid to liquid is more involved since there are many levels of organization of molecules that occurs. When injected at high pressure and temperature, they because ductile and flow into the mold, and upon cooling they develop polymer chains that cross-link and weave together to make a solid that retains its shape. The plastic solid becomes a brittle glass if it is cooled to low enough temperatures.

Most plastics do emit and consume vapors from the surrounding air, such as releasing hydrocarbon-based vapors (yielding a "plastic smell") and reacting with oxygen in the air (old school polyethylene-based plastics become oxidized and brittle over time). They also can release or absorb water vapor. This is why the weight of discs can change over time, depending on the conditions (temperature, humidity, sealed vs open, etc., etc.).

It's true that you have to stick with a given blend of plastic in order to be able to reliably predict its behavior. The shrinkage is governed by the thermal expansivity (times the temperature difference between the working temperature in the injection molding machine and the ambient atmosphere), and its deformation is controlled by its elastic moduli. The rim contracts inward. Sometimes the flight plate shrinks with it, and sometimes it resists the shrinkage and domes up. The molds themselves have no dome, any dome you see is produced by shrinkage.

Whether or not resistance or doming from the flight plate causes the wing to bend up or down is also quite variable. This depends mostly on the flight plate thickness and the elastic constants of the plastic, and also how it is handled by the molding operator. Sometimes the flight plate is more rigid, and may tend to bend the wing down upon cooling, making the disc more understable. Sometimes the flight plate may be less rigid, allowing the rim to shrink while retaining a high parting line. The dome itself exerts a force that depends on the dome curvature and the elastic rigidity of the plastic, which is highly variable, so there are no simple rules that work in every case.